Breathing circuits are utilized to conduct inspiratory gases from a source of same, such as from an anesthetic machine, to a patient, and to conduct expiratory gases away from the patient. The gases are conducted through two or more conduits, and, generally, at least a portion of the expiratory gas is recycled to the patient after removal of carbon dioxide. To facilitate description of the prior art and the present invention, the end of a conduit directed toward a patient shall be referred to as the distal end, and the end of a conduit facing or connected to a source of inspiratory gases shall be referred to as the proximal end. Likewise, fittings and terminals at the distal end of the breathing circuit, e.g., connecting to or directed at the patient airway device (i.e., endotracheal tube, laryngeal mask, or face mask), will be referred to as distal fittings or terminals, and fittings and terminals at the proximal end of the breathing circuit will be referred to as proximal fittings and terminals. For further information on breathing systems, and anesthetic and ventilation techniques, see U.S. Pat. No. 3,556,097, U.S. Pat. No. 3,856,051, U.S. Pat. No. 4,007,737, U.S. Pat. No. 4,188,946, U.S. Pat. No. 4,232,667, U.S. Pat. No. 5,284,160, Austrian Pat. No. 93,941, Dorsch, J. A., and Dorsch, S. E., Understanding Anesthesia Equipment: Construction Care And Complications, Williams & Wilkins Co., Baltimore (1974) (particularly chapters 5-7), and Andrews, J. J., "Inhaled Anesthetic Delivery Systems," in Anesthesia, Fourth Edition, Miller, Ronald, M. D., Editor, Churchill Livingstone Inc., New York (1986), (particularly pp. 203-207). The text of all documents referenced herein, including documents referenced within referenced documents, is hereby incorporated as if same were reproduced in full below.
U.S. Pat. No. 4,265,235, to Fukunaga, describes a unilimb device of universal application for use in different types of breathing systems, which provides many advantages over prior systems. The Fukunaga system utilizes a space saving coaxial, or tube-within-a-tube, design to provide inspiratory gases and remove expiratory gases. Generally, the inner tube is connected at its proximal end to a source of inspiratory, fresh gas, while the outer tube proximal end is connected to an exhaust port and/or to a carbon dioxide absorber (the latter at least partially exhausts into the inspiratory gas source when used in a circle system). In addition to reducing the size of the breathing apparatus connected to a patient by reducing the number of tubes near the patient, the Fukunaga system has additional benefits, such as serving as an artificial nose (expired air warms and humidifies inspired air as the opposing two flows arc co-axial in the unilimb device). The Fukunaga circuit is also safer than prior co-axial systems, since the distal end of the inner tube is not connected to the outer tube at a distal fitting, so that the outer tube can be axially extended with respect to the inner tube without disconnecting the proximal end of the inner tube from the source of inspiratory gases; this safety feature can also be used to increase the dead space between the distal ends of the inner tube and outer tube, and thereby allow for adjustment of the amount of expiratory air the patient rebreaths. Dead space is defined herein as the part of the breathing circuit external to the patient which, at the end of expiration, is filled with exhaled gases to be inhaled at the next breath (generally the expired air in the dead space is combined with oxygen and/or other gases provided from a source thereof).
An embodiment of the Fukunaga unilimb device is commercially manufactured as the UNIVERSAL F.TM. by King Systems of Noblesville, Ind., USA. The device includes a proximal terminal comprising a hollow, T-shaped housing with three ports: an inspiratory gas port, an expiratory gas port at a perpendicular angle to the inspiratory gas port, and a third ("patient") port. The proximal terminal is connected to an outer tube and a coaxial inner tube, which carry gases to and from the proximal terminal. The outer tube is flexible and corrugated, and formed of a transparent (or semi-transparent) material. The proximal end of the outer tube is sealably connected and bonded to the patient port of the proximal terminal. The proximal end of a dark colored, flexible inner tube is sealably connected and bonded to the inspiratory port, and extends through the T-shaped housing, out the patient port, and passes through most of the axial length of the outer tube. The dark color of the inner tube readily permits the user to see through the outer tube to determine whether the inner tube is properly connected.
The inner diameter of the outer tube is sufficiently larger than the outer diameter of the inner tube to permit adequate patient respiration. The distal end of the outer tube is sealably connected and bonded to the exterior of an annular housing which forms a distal terminal. The annular housing of the distal terminal is designed to prevent the distal end of the inner tube from extending beyond the distal end of the outer tube. The entire unit is designed for disposal after a single use.
The UNIVERSAL F.TM. device offers great advantages over prior dual line and unilimb anesthesia circuits, and respiratory assist devices. However, manufacture of the entire unit requires several complex steps, and must be done with care so that the inner and outer tubes are properly scaled and bonded to the proximal terminal ports at their proximal ends; it is particularly important that the inner tube proximal end be firmly connected to the proximal terminal when the inner tube carries inspiratory gases, since disconnection during use may not allow sufficient oxygen and/or anesthetic gases to reach a patient, which is highly undesirable.
While U.S. Pat. No. 4,265,235, to Fukunaga, teaches that the tubes and terminals of such a unilimb device can be detachable from one another, in practice, the proximal end of the inner tube is firmly bonded to the inspiratory port, since there remains a risk that the proximal end of the inner tube could be disconnected from the inspiratory port during use if a pressure fit (or friction fit) alone is used. Even if detachment of the inner tube is detected, the design of prior art unilimb devices does not facilitate the reconnection of the inner tube to the inspiratory port of the proximal terminal due to the need to pass the inner tube proximal end through the length of the proximal terminal via the patient port so that it can reach and be connected to the inspiratory port. Thus, the unilimb devices currently used generally comprise a proximal terminal having an integrally connected inner tube and outer tube.
Due to its single-use design, the entire unilimb device, including the distal terminal, proximal terminal, inner tube and outer tube, is disposed of after a single use, along with multiple devices usually connected to the patient nozzle, such as a CO.sub.2 monitor (capnometer), temperature and humidity controlling and monitoring devices, an O.sub.2 controlling and monitoring device, and an infection controlling device (e.g., a filter). This, in addition to the inconvenience of requiring fittings (or a housing accommodating same) for these additional devices at the patient nozzle or distal terminal, replacement of these fittings, tubing, and devices after a single use is expensive, and contributes to ever-growing medical wastes, which are sometimes difficult to find disposal sites for. All of the systems described in the aforementioned patents suffer from similar deficiencies. Therefore, there is a need for an improved unilimb device and ventilation system which reduces costs and helps the environment by reducing waste. There is also a need to simplify the construction, and to increase the safety, efficacy, and reliability of such devices.
Breathing systems generally provide oxygen to a patient, while removing carbon dioxide produced by the patient. For example, in anesthesia, or intensive care, the patient is provided an artificial breathing atmosphere, in which the physician provides a mixture of gases to the patient. In addition to providing oxygen and a variety of vaporized anesthetic agents to the patient, the physician may permit the patient to rebreathe some expired gases. Rebreathing simply consists of inhaling gases which have been expired, including carbon dioxide. However, assisted respiration/ventilation to a patient must be safe, and hypoxia (i.e., patient oxygen deficiency) must be avoided. Therefore, inspiratory gases are generally provided at high enough pressure, tidal volume and respiratory rate (hyperventilation) to ensure that hypoxia and atelectasis (lung alveolar collapse) is avoided. Thus, patients are given very high levels of oxygen to avoid hypoxia, but unfortunately they often experience abnormally low carbon dioxide levels (i.e., hypocarbia or hypocapnia), and insufficient carbon dioxide can have a negative impact on vital organs (e.g., brain, heart, splanchnic organs, etc.). However, many physicians believe that increasing arterial carbon dioxide partial pressure (P.sub.a CO.sub.2, also referred to as arterial carbon dioxide tension, often reported as mmHg) in patients by increasing the carbon dioxide breathed by the patient (e.g., by increasing the amount of rebreathing) would cause hypoxia. Thus, it was believed that hypercapnia during assisted ventilation was harmful, since it was believed it would be associated with hypoxia. Further, hypocapnia, while it can be harmful, was believed to be less harmful than hypoxia. Therefore, there remains a need for an improved artificial ventilation method which controls or avoids hypocapnia without compromising vital organ tissue perfusion or oxygenation (i.e., avoids hypoxia).